In the cold drawing of a metallic tube, lubrication treatment is performed in order to reduce the friction which occurs due to the contact of a mother tube, which is the material to be worked, with tools such as a die and a plug, thereby preventing the occurrence of seizing and vibrations/chattering. In general, in lubrication treatment, used is a method which involves forming chemical treatment lubrication films on the inner and outer surfaces of a mother tube. However, in obtaining a small-diameter longer-length tube by drawing, the mother tube is generally long enough, and hence in forming chemical treatment lubrication films on the mother tube, attention must be paid to sufficiently apply chemical treatment to the mother tube so as to fully cover the inner surface of the mother tube. For this reason, the treatment requires a large number of man-hours and chemical agents which are used are relatively expensive, resulting in an increase in operating cost.
A metallic tube made of a Ni-based high alloy is in heavy usage as a heat transfer tube in the steam generator of a nuclear power plant. In a mother tube made of a Ni-based high alloy, it is difficult to form chemical treatment lubrication films on the surfaces of the mother tube and, therefore, in the case where a metallic tube made of a Ni-based high alloy is produced by cold drawing, the operating cost required for the forming of chemical treatment lubrication films increases further.
Therefore, the forced lubricating drawing (the high-pressure drawing process) has been developed. The forced lubricating drawing is a kind of cold drawing in which lubrication treatment is directly performed by an oil lubricating film. The forced lubricating drawing stabilizes cold drawing and produces a great effect on the quality improvement in a drawn metallic tube.
Usually, the drawing of a metallic tube by the forced lubricating drawing is carried out by the following procedure:
(1) After filling a high-pressure container with a lubricating oil, the container holding a mother tube, which is a material to be worked and is inserted thereinto, the pressure of the lubricating oil is increased by a pressure booster.
(2) The lubricating oil thus pressurized forms lubricating oil films between the mother tube and tools such as a die and a plug, the die being tightly disposed to an open end of the high-pressure container, the plug being in place in a working position.
(3) With the inner and outer surfaces of the mother tube forcedly lubricated with the formed lubricating oil films, the mother tube is drawn and finished to prescribed dimensions determined by the tools, whereby a metallic tube is obtained.
With respect to drawing by this forced lubricating drawing, various proposals have hitherto been made and for example, there are Patent Literature 1 and Patent Literature 2. Patent Literature 1 relates to a forced lubricating drawing apparatus used in the forced lubricating drawing. The forced lubricating drawing apparatus proposed in Patent Literature 1 comprises: a high-pressure container whose leading end is tightly secured to the back face of the die and which houses the mother tube; a plug supporting bar which is axially movably held in the high-pressure container; and a device which supplies a lubricating oil into the high-pressure container.
A forced lubricating drawing apparatus of such a configuration has such a telescopic construction that a foremost end portion of the high-pressure container can be elongated or shortened axially, while a movable part of the foremost end of the high-pressure container is configured such that the front outside diameter thereof is smaller than the rear inside diameter thereof, with the result that the movable part is able to push the back face of the die by the lubricating oil pressure in the high-pressure container, wherein the whole high-pressure container can be displaced to a mother tube insertion position as being off the drawing line. For this reason, in the drawing method using the forced lubricating drawing apparatus described in Patent Literature 1, it is claimed that a mother tube can be readily and positively subjected to drawing by the forced lubricating drawing.
Patent Literature 2 proposes a method of producing a small-diameter longer-length tube by cold working by use of the forced lubricating drawing in which at least final cold working as involving wall thinning is carried out by plug drawing with a high-pressure lubricating oil of not less than 500 kgf/cm3 in pressure. In Patent Literature 2 it is claimed that at least final cold working as involving wall thinning is performed by the forced lubricating drawing using a high-pressure lubricating oil, whereby dimensional variations along an axial direction of tube can be reduced without the occurrence of seizing in a resultant metallic tube.
In the case where a metallic tube used as a heat transfer tube in a steam generator is produced, in general, inspection by an inner probe type eddy-current flaw detection is conducted for inner surface defects of a metallic tube. In the drawing method of a metallic tube described in Patent Literature 2, it is claimed that because dimensional variations along a tube axial direction of an obtained metallic tube are small enough, the noises caused by dimensional variations of a metallic tube in the inner probe type eddy-current flaw detection is suppressed and hence inner surface defects can be strictly detected on the basis of outputs of a flaw detection device.
Lubrication is performed by forcedly forming lubrication oil films between a mother tube and tools using the drawing method by the forced lubricating drawing described in Patent Literature 1 or 2, whereby in many cases it is possible to prevent the seizing between the tools and the metallic tube. However, the seizing may sometimes occur even when the drawing method by the forced lubricating drawing described in Patent Literature 1 or 2 is used. In addition, in the case where a mother tube made of a Ni-based alloy is subjected to drawing, vibrations/chattering may sometimes occur due to the friction occurring between the plug and the mother tube.
Furthermore, in the drawing by the forced lubricating drawing, in some cases, a lubricating oil is locally trapped on the inner surface of the mother tube and minute recessed portions are formed, resulting in the occurrence of defects called oil pits. If such oil pits are formed in drawing, the inner surface roughness of an obtained metallic tube deteriorates.
On the other hand, with respect to the lubricating oils used in cold drawing, various proposals have hitherto been made, and there is Patent Literature 3, for example. Patent Literature 3 describes a lubrication method in which a wire, a rod or a tube blank made of carbon steel or alloy steel is subjected to acid pickling, a lubricating oil is then applied, and cold drawing is performed. On this occasion, the lubricating oil which is used is a lubricating oil which is adjusted with a thickening agent so that the viscosity becomes 100 to 3000 centipoises at 20° C. by mixing 5 to 40 parts of dialkyl polysulfide containing not less than 30 wt % of sulfur and 20 to 70 parts of one kind or two or more kinds selected from the group consisting of organic compounds containing not less than 15 wt % of sulfur.
In the lubrication method for cold drawing described in Patent Literature 3, it is claimed that by using the above-described lubricating oil, it is possible to perform drawing without the formation of a chemical treatment lubrication film on a material to be worked, that it is possible to reduce the operating cost required by lubrication treatment, and that the surface finish of the material to be worked after drawing is excellent. However, Patent Literature 3 relates to cold drawing which involves applying a lubricating oil at normal pressure and no study is made on the cold drawing by the forced lubricating drawing using a lubricating oil whose pressure is increased.